Under continuing study are the ways in which circulating hormones, neurohumors, and neurohormonal substances influence behavior and, in particular, how they integrate functions of organ systems by redistributing the cardiac output. Specifically, this study concerns itself with how peptides of pituitary, hypothalamic, and other brain origins modify blood flow to various regions of the brain and other nervous system structures. It will be coordinated with behavioral studies conducted by collaborators in order to focus attention on areas of the brain which are likely to be involved in characteristic behavioral responses to these peptides. A relatively simple and rapid method is employed permitting the measurement of nutritional (exchangeable) flow of blood through many areas of the nervous system at the same time in small laboratory animals without the use of anesthesia or restraint. The method has now been refined to permit measurements of perfusion together with another index of function, cyclic nucleotides in regions as small as 50 micron m3.